A fuel cell cogeneration system cannot generate electricity unless the temperature of the fuel cell is raised to a predetermined temperature and thus needs to be preheated during starting. In the related art fuel cell cogeneration system, a heater is used to heat cooling water by which the temperature of the fuel cell is then raised. However, the temperature raising method using a heater has a disadvantage that the electric power consumed during the starting of the fuel cell cogeneration system is great. Thus, a method has been proposed which comprises utilizing hot water stored in a stored hot water tank to raise the temperature of the fuel cell during starting (see, e.g., JP-A-2002-42841). The configuration of this method is shown in FIG. 4. A fuel cell 11 undergoes reaction of a hydrogen-rich gas (hereinafter referred to as “fuel gas”) supplied and an oxidizer gas such as air to generate electric power and heat. The fuel gas is produced by heating a starting material such as natural gas in an atmosphere containing water vapor in a fuel processing means 21. The oxidizer gas is supplied into the fuel cell 11 by an air supplying unit 41.
Cooling water of supplying heat during starting to raise the temperature of the fuel cell 11 and removing the heat generated during electricity generation to maintain the fuel cell 11 at a predetermined temperature is circulated in the fuel cell 11 by a cooling water circulating pump 12. The cooling water which has passed through the fuel cell 11 then exchanges heat with stored hot water in a cooling water heat exchanger 13. In a stored hot water tank 31 is stored hot water of recovering the heat generated by the fuel cell system.
During electricity generation, stored hot water is taken out of the stored hot water tank 31 at the bottom thereof by a waste heat transporting means 39 through which it is sent to a stored hot water pathway 15, and then passed through the cooling water heat exchanger 13 where it then exchanges heat with cooling water flowing through a cooling water pathway 16. The stored hot water which has thus recovered heat is then returned to the top of the stored hot water tank 31 by the waste heat transporting means 39.
During starting, the pathway from the stored hot water 31 to the stored hot water pathway 15 is switched by the waste heat transporting means 39. In other words, stored hot water is taken out of the top of the stored hot water tank 31, and then passed through the cooling water heat exchanger 13 where it supplies heat to cooling water, and the stored hot water which has thus supplied heat to cooling water is then returned to the bottom of the stored hot water tank 31. By thus transferring the heat of the stored hot water tank 31 to cooling water, the temperature of the fuel cell 11 is raised. By raising the temperature of the fuel cell 11 with hot water in the stored hot water tank 31 in the aforesaid fuel cell cogeneration system, the electric power consumed during starting can be reduced.
Further, as another example, it has been proposed that an exhaust gas heat exchanger 23 of recovering heat from the fuel processing means 21 is provided upstream the cooling water heat exchanger 13 on the stored hot water pathway 15, so that the heat recovered from exhaust gas is transferred to cooling water via the cooling water heat exchanger 13 to accelerate the rise of temperature of the fuel cell as in a fuel cell cogeneration system shown in FIG. 5 (see, e.g., JP-A-2002-25591).
However, the fuel cell cogeneration system shown in FIG. 4 is disadvantageous in that it takes much time to raise the temperature of cooling water when the difference in temperature between the stored hot water and the cooling water which exchange heat with each other is small.
Further, even when only the heat recovered from exhaust gas is utilized to raise the temperature of the fuel cell as in the fuel cell cogeneration system shown in FIG. 5, the time required to start the fuel cell cannot be sufficiently reduced. Thus, a configuration comprising one shown in FIG. 4 and one shown in FIG. 5 in combination can be proposed, but even this case is disadvantageous in that the temperature of the fuel cell cannot reach a desired operating temperature or, even if the operating temperature is reached, the time required to start the fuel cell cannot be sufficiently reduced.